affected by the internal heat beyond the merest fraction of a degree. The earth is also surrounded by an aërial envelope or atmosphere, consisting in the main of 79 parts nitrogen and 21 oxygen, with a fractional percentage of carbonic acid; and this gaseous envelope, being an elastic medium, is denser at the earth's surface (where it presses with a weight of 14 lb. on the square inch), and becomes rarer and rarer as we ascend in space, till at the height of 45 or 50 miles its presence becomes inappreciable. This atmosphere is the medium through which the sun's light and heat are equally diffused; the laboratory in which all meteorological phenomena-winds, rains, clouds, storms, &c.—are elaborated; and, in fine, the source whose varied mutations are the proximate causes of all climatic diversity. In treating of the earth, various terms and technicalities are necessarily employed by geographers, and with these the student should early render himself familiar. Thus, the imaginary line on which the earth turns in her daily rotation from west to east is termed the axis; the extremities of this axis, north and south, the poles; the equator, the circle between the two poles which cuts the globe into two equal portions or hemispheres; the torrid, temperate, and frigid zones, the varying belts of surface temperature as we proceed northward and southward from the equator, where the sun's heat is greatest; the ecliptic, the apparent path of the sun round the earth; meridians, those lines drawn from pole to pole over the earth's surface, and at right angles to the equator, along which the sun is vertical at his highest daily ascension or noon; latitude, the distance of any place, measured in degrees, north or south of the equator; and longitude, the distance of the same place east or west from any arbitrary or convenient meridian-that in Britain being the meridian of Greenwich; that in France, Paris; and that in Germany, the Faroe Islands. To the student who wishes to enter more minutely into the consideration of the earth's planetary or astronomical relations, we may recommend persual of Sir J. Herschel's 'Outlines of Astronomy,' Mr G. D. Chambers's 'Handbook of Descriptive and Practical Astronomy;' and Mrs Somerville's 'Connection of the Physical Sciences.' For a fuller explanation of the technical terms of his science, he may consult the Glossary appended to the present volume. III. THE EARTH-ITS INDIVIDUAL STRUCTURE AND COMPOSITION. The Rocky Crust-Its Constitution and Formation. 24. HAVING considered in the preceding chapter some of the more obvious relations of the globe as a member of the Solar system, and the terms usually employed to express their connections, we now proceed to describe the leading features of its own special structure. As the knowledge of its general relations was derived chiefly from Astronomy, so a knowledge of its individual structure is mainly obtained from the teachings of Geology. As the one set of facts could be taught without going deeply into the problems of the astronomer, so the other may be understood without entering largely into the reasonings of the geologist. What, for instance, is the nature of the earth's rocky crust? How are its rocks arranged, and how does this arrangement affect its superficial character? What has stretched out the level plain and upheaved the rugged mountain? What renders one soil obnoxious and sterile, and another genial and fertile? And as the earth's crust is continually undergoing modification under the operations of external and internal forces, what the effects of such modifications on the general geography of the globe? These and similar problems geology endeavours to solve, and, with a little explanation, these solutions may be rendered intelligible to the student of Physical Geography. 25. Geologists speak of the "crust of the globe" just as the housewife talks of the "crust of her loaf." The crust of the loaf is one thing, the inside of it another. The crust or exterior portion of the earth, composed of rocks and rock-materials that can be seen and handled, is one thing; the interior, of which we can know nothing by direct observation, may be, and in all likelihood is, a very different thing. By observation and comparison we can determine a great many truths respecting the structure and composition of the one; respecting the other we can offer at best C little more than conjecture. This outer portion or rocky crust is the great theatre of all geographical phenomena-the foundation of the land and waters-the arena of climatic influences-the field of vital development; and, as such, it behoves the geographer to know something of its history and constitution. That history, as geology has taught us, is a long and varied one; that constitution, as bearing more immediately upon the problems of geography, we shall endeavour briefly to explain. 26. On the most cursory inspection of quarries, railway-cuttings, sea-cliffs, and ravines, the observer will find a great portion of the rocks arranged in layers, or lying one above another in beds or strata (Lat. stratum, spread out). These are said to be stratified, and generally consist of sandstones, clays, shales or consolidated muds, limestones, and other similar rocks. He will also find another set not spread out in layers, but rising, hard and massive, in no determinate arrangement. These are termed the unstratified, and consist of such rocks as the granites, greenstones, basalts, and lavas. How, he will naturally inquire, have two sets of rocks so dissimilar in character and arrangement been produced? And as we can only interpret nature's productions by a knowledge of nature's operations, we must seek for the answer to this question in what is now taking place around us. And first, if we turn to any lake, estuary, or bay of the ocean, we will find that the mud, sand, and gravel carried down by the rivers or washed from the cliffs by the waves and tides, are deposited and arranged along the bottom of these receptacles in layers or strata more or less horizontal, and in course of ages one above another, precisely like the shales and sandstones of the quarries and sea-cliffs. Here, then, we are entitled to infer that rocks arranged in layers or strata have been formed through and by the agency of water (Lat. aqua); that is, have been deposited as sediment (Lat. sedere, to settle down) in water, or brought together and assorted by the action of moving water; and hence they are termed aqueous, sedimentary, or strati B A A A, Stratified; BB, Unstratified Rocks. fied. If we turn, in the next place, to the volcano or burning mountain, we find that lava and other molten rock-matters are discharged from its crater; and these, when cooled and consolidated age after age, form mountain-masses, and fill up chasms and rents produced by earthquakes, precisely as the granites and green stones and basalts do among the stratified rocks with which they are associated. Here, again, we are entitled to infer that the greenstones, basalts, and other similar rocks are the products of fire (Lat. ignis, fire), and hence they are designated igneous, eruptive, or unstratified. 27. In the crust of the earth, then, we have two main sets of rocks the stratified and the unstratified; the one formed through and by the agency of water, the other through and by the agency of fire. The former are chiefly the products of aqueous and atmospheric waste, the latter the products of igneous reconstruction; and between these two forces, the aqueous and igneous, the crust of the earth has ever been held in habitable equilibrium. Were the aqueous and atmospheric forces to operate uncontrolled, all the higher portions of the dry land would in the course of ages be worn down, and the whole reduced to a dreary uniformity of level; but, to prevent such a contingency, the volcanic forces are perpetually exerting themselves from below, and once more upheaving the crust into dry land and diversity of surface. What the frosts and rains and rivers are wearing and carrying from the higher lands is deposited in layers of mud, sand, and gravel in the lakes and estuaries below; and what is wasted from the sea-cliffs by waves and tides is borne along and distributed in the bays and other sheltered recesses of the ocean. These deposits, in course of time, become consolidated, by pressure and internal chemical changes, into shales and sandstones and conglomerates; and we can readily conceive the low, level, superficial aspect of a globe where this sedimentary process went on unchecked for ages. But the forces from within are as incessantly at work as the forces from without, and sooner or later (according to some law whose order is yet unknown) these strata are upheaved into dry land, with all that diversity of surface which seems inseparable from the efforts of the earthquake and volcano. The great design of creation seems clearly to be diversity in time as well as diversity in space. Whether under the open air or under the waters of the ocean, hill and dale, level plain and rugged mountain, are ever attended by diversity of soil, diversity of winds and currents, heat and cold, and all the other climatic influences on which diversity of plantlife and diversity of animal life are so intimately dependent. To the primary geological oscillations of the earth's crust are we, therefore, indebted for all that confers on its surface geographical variety and diversity; and hence the value of some knowledge of geology to him who would thoroughly comprehend the existing aspects of nature. Relative Age and Arrangement of Rock-Formations. 28. If, then, the crust of the globe be in a state of oscillation between the aqueous and atmospheric forces that waste and wear from without, and the igneous forces that reconstruct and upheave from within-if cliffs and hills are worn down, and lakes and estuaries filled up and converted into alluvial plains—if plains are thrown up into mountains, and the sea-bed into dry land—if large tracts of the earth are gradually raised higher and higher above the ocean, while other regions are gradually submerged-it is clear that different portions of the rocky crust must be of different ages, and composed of different materials. The present distribution of sea and land has undergone many noted modifications even within the historic era; and if such changes have been accomplished within a period so brief as a few thousand years, what may we believe to have taken place during the thousands of ages that preceded human history? So numerous have been these changes, as clearly demonstrated by geology, that every portion of the existing dry land has been repeatedly beneath the waters; and that which now constitutes the bed of the ocean, has in like manner been the dry land of former epochs. "There rolls the deep where grew the tree; The hills are shadows, and they flow Like clouds, they shape themselves and go!" The record of these changes lies in the stratified rocks, each period producing its own sedimentary deposits, and these deposits constituting what geologists term a formation, as having been formed at a certain period, and under the peculiar conditions of that period. Farther, as the sediments of existing lakes and estuaries and seas imbed the remains of plants and animals that have lived in these areas, or have been drifted from the land-leaves, branches, trunks, bones, teeth, shells, and crusts; so the sediments of former ages contain the remains of plants and animals that then existed -these remains being petrified, or converted into stone. These petrifactions, or fossils, as they are termed, differ widely in many instances from the plants and animals now peopling the globe; and this difference may be said to increase with the depth and age of the strata that contain them. As a general rule, the older strata |